Model design is generally driven by these factors:

1. What are the dimensions of the wind tunnel?

The maximum dimensions of the tunnel will determine the wingspan and maximum AoA the vehicle can be tested at. Blockage and tip clearances also need to be considered.

2. How high is the expected dynamic pressure in the wind tunnel?

All aerodynamic forces are directly related to the freestream dynamic pressure, with a higher dynamic pressure meaning higher forces and moments.

3. What materials and equipment do I have available to me and at what cost?

If I have a 6 axis CNC machine capable of machining aerospace grade aluminium this will be more expensive that a desktop 3D printer.

4. What is the manufacturing time window I have?

Workshops will have a lead time, what is it? And does it fit in with my project timeline? Other equipment which is available for all to use may have queues.

5. Are there size restrictions?

Most machines will have some sort of size restriction. For example a 3D printer will have a finite build volume and a CNC machine will have a maximum billet size. This will determine the number of pieces your model will have.

6. What is the maximum blockage I am satisfied with and at what test condition is this satisfied?

High AoA tests will have a higher percent blockage than low AoA tests.

7. What do I have to mount my model to?

How large is the loadcell? Do I need to recess any instrumentation inside my model?

8. Are there any unique features of the vehicle which may contribute to issues?

If I have a large internal duct running through my vehicle, space will be limited inside.

The main consideration is that all these restrictions are interconnected and will each determine the model size. The following example shows how multiple design constraints drive the final model design (note numbers are for illustration purposes only and have not been calculated accurately):

1. I would like to test a traditional tube and wing aircraft in my 4 foot x 3 foot wind tunnel. To avoid needing tip corrections in the tunnel, I would like a model with a maximum of 70% of the tunnel width.

2. The dynamic pressure at 60 m/s is approximately 2200 Pa.

3. Wing root bending moments of 10 Nm at the maximum AoA of 15 degrees and the test dynamic pressure.

4. 3D printed plastic will fail at 5Nm, but aluminium will fail at 12 Nm. A plastic model would require wing spars and an aluminium model will not.

5. Machined aluminum costs 15x more than plastic and required 10x longer to manufacture.

6. Wing spars will need to be at least 10mm diameter, which is 1mm thinner than the thinnest aerofoil profile on the wing.

7. The loadcell plate requires a minimum width of 75mm and the fuslage at the 70% span scale has a diameter of 60 mm

8. Blockage at 15 degrees AoA is 15%

9. There are two wing mounted propellers requiring power, but batteries cannot fit inside the model. A power supply can be used, but wires will need to be passed through the test section and into the model.

10. The maximum pitching moment the load cell can take is 5Nm. If I have the loadcell mounted at 50% of the vehicle the maximum moment will be 5Nm, at 70% of the vehicle it will be 7Nm.

Will everything fit and is the tunnel suitable for this test? Extreme care needs to be taken when planning for and building models.

The top image shows an exploded view of the Agard B model in CAD. The video shows a time lapse of it being assembled using araldite and wooden dowel connectors (not shown in image, however the alignment holes are). The parts have been 3D printed and the vehicle is split along multiple planes. All holes for bolts and alignment of pieces have been printed and then drilled out.

If you have trouble viewing the video, please try another browser (Microsoft Edge seems to work best)